Quantum Computing Milestone: Researchers Compute With ‘Hot’ Silicon Qubits

Two research groups say they've independently built quantum devices that can operate at temperatures above 1 Kelvin-15 times hotter than rival technologies can withstand.

The ability to work at higher temperatures is key to scaling up to the many qubits thought to be required for future commercial-grade quantum computers.

A team led by Andrew Dzurak and Henry Yang from the University of New South Wales in Australia performed a single-qubit operation on a quantum processor at 1.5 Kelvin. Separately, a team led by Menno Veldhorst of Delft University of Technology performed a two-qubit operation at 1.1 Kelvin. Jim Clarke, director of quantum hardware at Intel, is a co-author on the Delft paper. Both groups published descriptions of their devices today in Nature.

HongWen Jiang, a physicist at UCLA and a peer reviewer for both papers, described the research as "a technological breakthrough for semiconductor based quantum computing."

In today's quantum computers, qubits must be kept inside large dilution refrigerators at temperatures hovering just above absolute zero. Electronics required to manipulate and read the qubits produce too much heat and so remain outside of the fridge, which adds complexity (and many wires) to the system.

At the higher temperatures described in the new research, control electronics could be placed right next to the qubits on the same chip. Instead of requiring dilution refrigerators that use isotopes helium-3 and helium-4, the system could be cooled using only helium-4. That should reduce the costs of building quantum systems-Dzurak describes the potential difference as going from a few million US dollars to a few thousand.

The devices reported in Nature compute using silicon spin qubits. These qubits are particularly appealing to semiconductor makers such as Intel because devices based on them could be produced using modern semiconductor manufacturing techniques.

"To me, these works do represent, in rapid succession, pretty big milestones in silicon spin qubits," says John Gamble, a peer reviewer for one of the papers and a senior quantum engineer at Microsoft. "It's compelling work."

Each silicon spin qubit consists of a few electrons held within a quantum dot. These quantum dots (which are different from the sort used in displays and cameras) are tiny wells or divots in silicon that lay just beneath the gate electrode of a conventional transistor. As charge flows through the transistor, electrons drop into the well, and electrostatic forces hold them in place.

Photo: Paul Henderson-Kelly/UNSW UNSW's Henry Yang (left) and Andrew Dzurak (right) have demonstrated a single-qubit operation at 1.5 Kelvin.

Several electrons held together in a quantum dot make a silicon spin qubit. The UNSW team created two qubits consisting of three electrons each for their experiment, and the Delft group made two qubits containing one and five electrons each. To compute with them, the UNSW team applied an AC electric field, while the Delft team used an AC magnetic field to manipulate the electron spins, causing the spins to point up (1), down (0), or in both directions at once-a quantum state known as superposition.

The Delft team demonstrated a two-qubit operation, in which the spins of two qubits are closely linked, such that an operation on one qubit can be controlled by the state of another qubit. This makes it possible to program logic across a series of qubits.

"If you have individual qubits, you can make as many as you want," says Veldhorst. "But it's only the interaction between them that allows you to do something useful."

One key to computing with these qubits was to find a way to manipulate them and read out the results at higher temperatures than conventional quantum methods would allow. To achieve this, both teams employed a technique called Pauli spin blockade.

With this technique, electrons can be forced to occupy the same quantum dot only if their spins are opposite. If their spins match, the electrons stay put in their respective wells. This readout method, which has been applied to silicon spin qubits in previous experiments, is meant to enable computations across many qubits.

Moving forward, Gamble is interested to see if the research groups can scale their approach to include more qubits. He's encouraged by their efforts so far, saying, "The fact that we're seeing these types of advances means the field is progressing really well and that people are thinking of the right problems."

Lee Bassett, a physicist focused on quantum systems at the University of Pennsylvania, says the researchers will need to find a way to improve coherence times-a measure of how long qubits remain viable-at 1 Kelvin. Other quantum technologies record coherence times of about 100 microseconds, but the Nature papers described times of only a few microseconds.

Overall, though, Bassett says the results have boosted his confidence in silicon spin qubits as a promising path for practical quantum computers.

"Each time these silicon devices pass a milestone-and this is an important milestone-it's closer and closer to the inflection point," he says. "This infrastructure of integrated, silicon-based electronics could take over, and this technology could just explode."

While Intel is also studying superconducting qubits, Clarke says the company is now focusing most of its research efforts on silicon spin qubits. "The spin qubits look a lot like transistors," he says. "And Intel ships 400 quadrillion transistors a year."

Meanwhile, Google and IBM have invested heavily in superconducting qubits. D-Wave's commercially-available quantum annealers also use superconducting qubits. Microsoft is pursuing topological qubits and the Maryland-based company IonQ has bet on using trapped ions as qubits.

The UNSW work was supported in part by the company Silicon Quantum Computing Proprietary Limited in Australia. The Delft work was performed at QuTech, a research institute of Delft University of Technology and the Netherlands Organization for Applied Scientific Research.

Intel is also attacking the temperature problem from another angle with its recent debut of a cryogenic chip called Horse Ridge that moves some control electronics inside of a dilution fridge and closer to the qubits.

Clarke says the new advance "doesn't change our timeline" for achieving a practical quantum computer-"it just reinforces our original timeline." He adds, "Realistically, it's probably 8 to 10 years until we have quantum systems that can achieve something that would change your life or mine in a way that a classical computer couldn't."